Did you get this tested or do you want me to give it a go?
% uname -a
Darwin hyperstation 9.2.2 Darwin Kernel Version 9.2.2: Tue Mar 4 21:17:34 PST 2008; root:xnu-1228.4.31~1/RELEASE_I386 i386 i386
We did some testing, with mixed results. ShuX built on Leopard/PPC would not run on either Panther or Leopard/PPC, but does run on Tiger/i386. Rachel built a copy of her app Atlantis on Leopard/i386, and it runs fine on that, Leopard/PPC, and Tiger/i386, but not on Panther.
Both ShuX and Atlantis show the same problem on Panther - that appears to be a framework bug. When the support bundle is loaded, the Objective-C symbols from the framework aren't getting resolved, which blows everything up. We've been over the linker and compiler flags for the perl-5.8.1-Panther target a dozen times or more, and found nothing to explain the difference. I'm going to create a simplified test project to try to isolate that.
I think the problems with ShuX on Leopard are bugs in ShuX. Atlantis working fine supports that theory. If you have an app that's working with CB on Tiger, and doesn't use any of the included par bundles, that would be a great test. The new version is supposed to be a drop-in replacement for the current release - you should be able to just check out the latest svn version, build the "All" target with the "App-Embedded" build config, and drop the resulting framework into your app bundle's Contents/Frameworks.
There are caveats - the PAR bundles aren't building yet, and the lack of project templates means that new CB projects are difficult to create in Xcode 3. Dealing with those won't be hard though, once the framework itself is solid. Got to build the foundation before the roof... :-)